Constraint templates delete endpoint

[lsviben]

What this PR does / why we need it:
Adds a delete endpoint for CT

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #5894

Does this PR introduce a user-facing change?:

constraint templates delete endpoint for V2:
DELETE /api/v2/constrainttemplates/{ct_name}

[lsviben]

/retest

[zreigz]

/lgtm
/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 84488eb2e044327ba3ad28f74bdcbea7d46d56e2

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lsviben, zreigz

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• OWNERS [lsviben,zreigz]
• cmd/kubermatic-api/OWNERS [lsviben,zreigz]
• pkg/handler/OWNERS [lsviben,zreigz]
• pkg/provider/kubernetes/OWNERS [lsviben,zreigz]

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